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A study presented at the Society for Neuroscience meeting, in San Diego last week, shows people who start using marijuana at a young age have more cognitive shortfalls. Also, the more marijuana a person used in adolescence, the more trouble they had with focus and attention.

"Early onset smokers have a different pattern of brain activity, plus got far fewer correct answers in a row and made way more errors on certain cognitive tests," says study author Staci Gruber, assistant professor of psychiatry at Harvard Medical School.

Her study evaluated 29 non-smokers and 35 chronic marijuana smokers - 20 began smoking pot regularly before age 16 while 15 started smoking after age 16. All were about 22 years old when the study was conducted.

They were asked to perform a card-sorting task where they were shown four cards that differed in color, shape, and number.

While the smokers performed tasks quickly, they did not learn from their errors when corrected - a hallmark that the part of the brain that governs executive function is impaired, says Gruber.

Gruber says the smokers were also far more impulsive than the non-smokers. "The higher their impulsivity, the worse they performed their task," she says.

Gruber's research also looked at frequency of drug use. "If you smoke marijuana regularly prior to age 16, it turns out you smoke twice as frequently per week and three times as much in terms of grams per week," she says.

Teen brains are only about 80 percent developed and don't fully mature until their mid-20s, the researchers say; they add that each day, more than 4,300 teens try illegal drugs for the first time.

Marijuana is the most commonly used illegal drug in the U.S. with 25.8 million Americans ages 12 and older reporting at least one instance of abuse in 2008, according to the National Institute on Drug Abuse.

"Our results provide further evidence that marijuana use has a direct effect on executive function, and that both age of onset and magnitude of marijuana use can significantly influence cognitive processing," says Gruber. "Given the prevalence of marijuana use in the United States, these findings underscore the importance of establishing effective strategies to decrease marijuana use, especially in younger populations," she says.

"There's a myth that teen brains will bounce back, that they are really resilient, but in fact they may not be. It appears that they may be more vulnerable to drug use," says Frances Jensen, professor of neurology at Children's Hospital Boston. While a young brain is more "plastic" and able to learn, it can "maladapt," says Jensen.

With the ongoing debate over legalization of marijuana in some states, the research undersores the importance of considering guidelines for its use in much the same way guidelines have been established for alcohol and nicotine, Gruber says.

Related research presented at the conference also suggests young brains are more susceptible to other drugs:

- In one study with rats, adolescent amphetamine use permanently altered brain cells linked to memory and decision-making.

- Binge drinking during adolescence altered a stress response in rats in adulthood. Problems regulating stress are associated with behavioral and mood disorders.

- Adolescent rats are also more susceptible to cocaine abuse than adult rats, and are more sensitive to lower doses. Over a wide range of doses, adolescent rats learned how to get cocaine more readily than adults and took more of the drug overall. And when researchers made it harder to get, the teen rats worked two to three times harder to obtain it.